A. Field of the Invention
The invention relates to the production of hex-3-enedioate esters, and more particularly to the production of dialkyl hex-3-enedioate, an intermediate used in the production of adipic acid, from substituted butenes such as 1,4-dialkoxy-2-butene.
Adipic acid is produced at a worldwide capacity of nearly five billion pounds, most of which is used as a raw material for nylon 66 polymer production. Adipic acid is produced commercially by the air oxidation of cyclohexane to a cyclohexanone/cyclohexanol mixture (KA oil) which is subsequently oxidized to adipic acid with nitric acid. Adipic acid is also produced commercially from phenol by hydrogenation to cyclohexanol, the cyclohexanol being subsequently oxidized with nitric acid to adipic acid. Although adipic acid has been so produced for nearly forty (40) years, there are two major disadvantages in the current commercial processes. The air oxidation of cyclohexane must be carried out at low conversion rates in order to achieve high selectivity; and the recycling of large amounts of cyclohexane is potentially hazardous. Benzene which is the hydrocarbon source, is not expected to continue to be a low cost material.
U.S. Pat. No. 4,166,913 to ARCO describes production of adipic acid from carbon monoxide and butadiene. The ARCO process can be summarized by the following equations: ##STR1## The need for dehydrating agents and the potential safety hazard of the high pressure CO/O.sub.2 mixture under catalytic reaction conditions are significant disadvantages in the practice of the above production method.
A second known alternative to the production of adipic acid is described in U.S. Pat. Nos. 4,169,956 and 4,171,451 to BASF. The process can be summarized schematically as follows: ##STR2## A major disadvantage in this process is the extremely high reaction pressure required in the carbonylation step.
Previous attempts to dicarbonylate 1,4-disubstituted-2-butenes have been relatively unsuccessful. In the attempt described by Medema, D.; van Helden, R.,; Kohll, C. F. Inorg. Chim. Acta., 3 (1969) 155, page 20, it was reported that only 3% of 3-pentenoyl chloride was obtained in the attempt to dicarbonylate 1,4-dichloro-2-butene. No dicarbonylated product was observed. In the dicarbonylation attempt described by Imamura, S.; Tsuji, J.; Tetrahedron 25 (1969) 4187, it was observed that only about 10-37% of linear dicarbonylated products were obtained in a palladium chloride-catalyzed carbonylation of 1,4-diethoxy-2-butene; and that large amounts of by-products resulted from isomerization and hydrogenolysis of the starting material or reaction intermediates.
If a method for the production of an intermediate of adipic acid could be found based on the less expensive butenes, or derivatives thereof, without the problems of hydrogenolysis and isomerization described above, such a method would be a significant advance in the art and is an object of this invention.